1.0 General Background
In a typical cellular radio system, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a radio access network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Packet System (EPS) have completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to radio network controller (RNC) nodes. In general, in E-UTRAN/LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes, e.g., eNodeBs in LTE, and the core network. As such, the radio access network (RAN) of an EPS system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
1.1 Long Term Evolution (LTE)
The architecture of the LTE system is shown in FIG. 1. In LTE the downlink (e.g. communications from a base station to a wireless terminal) is based on orthogonal frequency division multiplexing (OFDM) while the uplink (e.g. communications from a wireless terminal to a base station) is based on a single carrier modulation method known as discrete Fourier transform spread OFDM (DFT-S-OFDM). See, e.g., Reference [2].
FIG. 1 shows that the E-UTRAN comprises base station (BS) nodes known as eNB nodes, which are connected to each other via the X2 interface. The eNB nodes are connected over the S1 interface to mobility management entities (MME) or serving gateways (S-GW). Both the S1 and the X2 interface may be divided into control plane (dashed lines) and user plane (solid lines) parts.
Admission control aims at ensuring that not more users than the cell can accommodate are admitted into the cell. Some users may have a negotiated service quality level, and such service quality levels should be maintained even when new users are admitted into the cell.
1.2 Load Balancing
Load balancing (LB) or “traffic steering” works by moving users one by one from highly loaded cells to less load cells in order to improve the user performance, e.g., user bit rate. A user being subject for load balancing, e.g., being located at the cell edge with relatively good link conditions to the neighboring cell, may be moved to the neighboring cell in order to balance the load between the two cells.
In another variation of the load balancing function, the handover margin (HOM) (see section 1.5 hereof) of the UEs in a cell may be altered in order to trigger measurements of candidate cells closer or further away from the serving cell depending on the load of the serving and candidate cells. UEs triggering such measurements are considered to be handed over to the reported candidate cell. The handover margin (HOM) may be regularly altered with the ambition to even out the load between cells, or changed when one cell is considered to be in an overload situation or fails to meet the quality of service (QoS) of served users, and a candidate cell is not in an overload situation. In this case the overload cell may hand over several UEs satisfying the handover margin (HOM) criteria to alleviate the overload situation.
1.3 Mobility Robustness Optimization
Mobility robustness concerns handover parameter adjustments. Essentially three situations should be avoided:                Too early handover, meaning that a UE is handed over to a candidate cell too early with a radio link or handover failure as a result. The UE returns soon to the source cell via re-establishment procedures.        Too late handover, meaning that the UE is handed over late to the target cell, so that the link to the source cell breaks before completing the handover.        Handover to wrong cell, meaning that the UE is attempted to be handed over to one target cell but the procedure fails, and soon thereafter the UE re-establishes at another cell. Most probably, it would have been better to have handed over the UE to the last target cell directly.        
A mobility robustness optimization (MRO) mechanism may adjust one or more of the following handover parameters controlling the event driven reporting of the UE:                A threshold indicating how much stronger a certain candidate cell needs to be before it is reported to the serving cell.        A filter coefficient applied to the measurement before evaluation triggers are considered.        A time to trigger, e.g., the time window within which the triggering condition needs to be continuously met in order to trigger the reporting event in the UE.        
For example, a higher ‘too early handover’ ratio than desired may be counter-acted by increasing the threshold, or delaying the report event trigger.
Another example, a higher ‘handover to wrong cell’ ratio than desired may be counter-acted by increasing the threshold towards the first, unwanted, target cell.
1.4 Handover (HO) Oscillation
An example of an oscillation is shown in FIG. 2. If T is less than a time period known as Tosc, then the HO is considered as an oscillation. The oscillation rate may be defined as the ratio between the number of oscillations and the total number of HOs. There is an upper boundary for an acceptable oscillation rate originating from e.g., core network load. Also the oscillation rate is related to end-user performance. On one hand oscillation are harmful as this induces additional signalling and delays, and on the other hand, oscillations allow the user to be connected to the best cell. This needs to be balanced in order for the end-user to experience the best performance.
1.5 Handover Margin and Time-to-Trigger
The handover margin (HOM) is the difference between the radio quality of the serving cell and the radio quality needed before attempting a handover. The radio quality is measured either using RSRP or RSRQ (see reference [5] for further explanation).
The UE triggers the intra-frequency handover procedure by sending a report known as an eventA3 report to the eNB. This event occurs when the UE measures that the target cell is better than the serving cell with a margin “HOM”. The UE is configured over RRC when entering a cell, and the handover margin (HOM) is calculated from the following configurable parameters:HOM=Ofs+Ocs+Off−Ofn−Ocn+Hys 
where:                Ofs is the frequency specific offset of the serving cell        Ocs is the cell specific offset of the serving cell        Off is the a3-Offset        Ofn is the frequency specific offset of the neighbor cell        Ocn is the cell specific offset of the neighbor cell        Hys is the hysteresis        
Thus the handover margin (HOM) may be changed by modifying one or more of these parameters. For inter-frequency handover a similar formula is used.
Time-to-trigger is the time period required before triggering a handover attempt. During this time the neighbor cell shall have better radio quality, and then the handover attempt is triggered.
The foregoing are further explained in reference [3].
1.6 Handover (HO) Cause
Once the serving cell has decided to perform a handover based on measurement reports received from the UE, the source cell performs a Handover Request over X2 AP, as basically shown in FIG. 3A. See also 3GPP TS 36.423, incorporated herein by reference.
In the HANDOVER REQUEST message, the source eNB must indicate the cause of the HO, which may be, e.g.:                Handover Desirable for Radio Reasons,        Resource Optimization Handover,        Reduce Load in Serving Cell        
Thus the target eNB knows that the handover is due to resource optimization or to reduce the load in the serving cell. A similar signalling may be routed via 51 links and the MME, see S1 AP, 3GPP TS 36.413, incorporated herein by reference.
1.7 Handover Preparation Failure
The target cell may be unable to admit the user intended for handover, and in such case the handover request may be denied by a Handover Preparation Failure, as illustrated in FIG. 3b. The cause in such a case may be one of the following:                No Radio Resources Available in Target Cell        Cell not Available        HO Target not Allowed        
1.8 RRC Connection Reestablishment
Following a Radio Link Failure (RLF) or a handover failure, the UE may perform RRC Connection Reestablishment, as shown in FIG. 4. See also 3GPP TS 36.331, incorporated herein by reference.
The UE shall set the UE-identity of the RRCConnectionReestablishmentRequest message as follows:                C-RNTI used in the source cell        physical cell identity (PCI) of the source cell        shortMAC-I        Using the above the last serving cell may identify the UE.        
1.9 Radio Link Failure (RLF) Report
Following an RRC Connection Reestablishment as a result of a radio link failure (RLF), the eNB may send a UElnformationRequest message to the UE, as shown in FIG. 5 [see also 3GPP TS 36.331, incorporated herein by reference].
If the eNB requests for an RLF report in the UElnformationRequest message and the UE has such information, then the UE provides the following information to the eNB:                The E-UTRAN Cell Global Identifier/Identity (E-CGI) of the last cell that served the UE (in case of RLF) or the target of the handover (in case of handover failure). If the E-CGI is not known, the PCI and frequency information are used instead.        E-CGI of the cell where the re-establishment attempt was made.        E-CGI of the cell that served the UE at the last handover initialization, i.e., when message 7 (RRC Conn. Reconf) was received by the UE, as presented in FIG. 10.1.2.1.1-1.        Time elapsed since the last handover initialization until connection failure.        An indication whether the connection failure was due to radio link failure (RLF) or handover failure.        Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) of the serving cell and the neighboring cells detected by UE at the radio link failure (RLF) event.        
Using the information above the eNB may deduce whether the radio link failure (RLF) was due to incorrect handover parameters (too early, too late) or due to a coverage hole (no cell with sufficient signal strength).
1.10 Radio Link Failure Indication
A purpose of the Radio Link Failure Indication procedure [see, e.g., 3GPP TS 36.423, incorporated herein by reference] is to transfer information regarding RRC re-establishment attempts between eNBs controlling neighbouring cells. The signalling takes place from the eNB at which a re-establishment attempt is made to an eNB to which the UE concerned may have previously been attached prior to radio link failure. A radio link failure (RLF) indication is illustrated in FIG. 6.
The eNB2 initiates the procedure by sending the RLF INDICATION message (shown by way of example in FIG. 6) to eNB1 following a re-establishment attempt from a UE at eNB2, when eNB2 considers that the UE may have previously been served by a cell controlled by eNB1.
The eNB2 may include the UE RLF Report (see Section 1.9 hereof) in the RLF INDICATION message, which may be used by the eNB1 to determine the nature of the failure.
The RLF INDICATION message sent from eNB B to eNB A may contain the following information elements:                Failure Cell ID: PCI of the cell in which the UE was connected prior to the failure occurred;        Reestablishment Cell ID: ECGI of the cell where RL re-establishment attempt is made;        C-RNTI: C-RNTI of the UE in the cell where UE was connected prior to the failure occurred;        shortMAC-I (optionally): the 16 least significant bits of the MAC-I calculated using the security configuration of the source cell and the re-establishment cell identity.        
1.11 Handover Report
An eNB initiates the procedure by sending an HANDOVER REPORT message to another eNB controlling neighboring cells, as generally illustrated in FIG. 7. By sending the HANDOVER REPORT message, eNB1 indicates to eNB2 that, following a successful handover from a cell of eNB2 to a cell of eNB1, a radio link failure occurred and the UE attempted RRC Re-establishment either at the original cell of eNB2 (Handover Too Early), or at another cell (Handover to Wrong Cell). The detection of Handover Too Early and Handover to Wrong Cell events is made according to TS 36.300.
The HANDOVER REPORT message typically includes the Handover Cause (see Section 1.6 hereof).
The Handover Report procedure is used in the case of recently completed handovers, when a failure occurs in the target cell (in eNB B) shortly after it sent the UE Context Release message to the source eNB A. The Handover Report procedure is also used in case of unsuccessful handovers, if the random access procedure in the target cell was completed successfully. The HANDOVER REPORT message contains the following information:                Type of detected handover problem (Too Early Handover, Handover to Wrong Cell);        ECGI of source and target cells in the handover;        ECGI of the re-establishment cell (in the case of Handover to Wrong Cell);        Handover cause (signalled by the source during handover preparation).        
2.0 Problems with Existing Technology
In deployments where mobility robustness optimization (MRO) and mobility load balancing (MLB) are running concurrently, there is a probability that mobility load balancing (MLB) actions impact mobility robustness optimization (MRO) statistics. If the MLB function moves a user between two cells due to load reasons and this user is faced with a radio link failure (RLF) or fulfills the oscillation criteria, then this information is included in the radio link failure (RLF) and oscillation statistics upon which mobility robustness optimization (MRO) is based. As such there is a risk that mobility robustness optimization (MRO) will react to the radio link failure (RLF) and handover oscillations caused by mobility load balancing (MLB). Moreover, the key performance indicators reported to the network management system are also affected and may present unfavorable mobility robustness optimization (MRO) performance.
Reference [3] discusses a method for avoiding conflicts between MRO and MLB. Reference [4] takes a more general approach to Self-Organizing Networks (SON) coordination. In 3GPP document 36.300 it is mentioned: The radio measurements contained in the RLF Report may be used to identify coverage issues as the potential cause of the failure. This information may be used to exclude those events from the MRO evaluation of intra-LTE mobility connection failures and redirect them as input to other algorithms. As such separation of radio link failures (RLF) due to coverage hole and due to inappropriate handover parameters is already known.
In a current approach, mobility robustness optimization (MRO), and other non-mobility causes impact radio link failure (RLF), handover failure (HOF), and HO oscillation rate, as illustrated in FIG. 9, since:                MRO is altering the conditions under which the UE is handed over to target cell        Load balancing is moving users between cells as a result of load imbalance        Other non-mobility causes, e.g., retracting users to prepare for cell maintenance or restart/reconfiguration and cell outage including compensation means, will change serving cell of users and thereby also mobility statistics. Moreover, admission rejects will cause failures not related to the mobility itself        